Author

Sieh, Kerry

Date of Issue

1984

Version

Published version

Abstract

Recent excavation and new radiocarbon dates of sediments at Pallett Creek are the basis for new
conclusionsr egarding the late Holocene history of the San Andreas fault. Systematicd issectiono f a
50-m-long, 15-m-wide, 5-m-deep volume of earth, centered on the fault, enables documentation in three
dimensions of fault patterns, lateral offsets, and vertical deformation associated with large earthquakes of
the past. The excavations expose evidence for 12 earthquakes that occurred between about 260 and 1857
A.D., with an average recurrence interval of about 145 years. Prehistoric slip events that occurred in
1720 ± 50, 1550 ± 70, 1350 ± 50, 1080 ± 65, and 845 ± 75 A.D. have lateral offsets that are comparable
to those of the most recent great earthquake of 1857. Thus all of these events represent earthquakes of
large magnitude. The lateral offsets of two other events, in 935 ± 85 and 1015 ± 100 A.D., are an order of
magnitude smaller and may be interpreted in several ways with regard to the size of these events. The
new data constraint he average recurrence interval for large earthquakes at this site to between 145 and
200 years but suggest a monotonic decrease in individual intervals to below this range during the past
900 years. On the basis of these data, the probability of a large earthquake with surficial fault rupture at
this site is between 0.2 and 5% during 1984 and 7 and 60% by the year 2000.